The present invention relates to x-ray tube design and x-ray tube power supply design. More particularly, the present invention relates to the development of a high efficiency x-ray source consisting of a fluorescent x-ray tube, and resonant power supply, which relies on plasma within the tube. The present invention further relates to the design of a power supply to achieve enhanced efficiency. This x-ray tube design can then be used in applications such as product irradiation, and more particularly the sterilization of materials such as foodstuffs and medical implements.
As public demand for greater safety from potentially harmful microorganisms increases, scientists must come up with more effective and efficient ways of providing safe products and environments. One technique that is well suited to the reduction in the quantities of microorganisms and pests is irradiation.
Irradiation uses relatively high doses of one of several forms of radiation, gamma rays, electron beam (e-beam), or x-rays, to kill microorganisms and pests that may be present in or on a given material. The radiation ionizes atoms that are sometimes part of critical molecules such as DNA and RNA. Damaging key cell components such as these causes the cells to die, and if enough cells are killed, then the organism dies. There are two main forms of irradiation in use today. They are gamma irradiation and e-beam irradiation. Gamma irradiation uses a radioisotope source such as cobalt-60 that emits gamma rays measured in the millions of electron volts (MeV), while e-beam uses an accelerator to accelerate electrons to MeV range energies. Although both technologies have performed well in limited situations, significant improvements are required to make this technology more accessible.
Gamma irradiation has the major drawback of using radioisotope sources. Radioisotopes cannot be turned off and therefore create a disposal hazard. Additionally, there is public perception linking all radioisotopes to atomic bombs and various accidental radiation deaths, as well as fear that the object being irradiated will be contaminated or somehow become radioactive, even if it cannot. All this makes it difficult to sell the public on the benefits on gamma irradiation. The high energy MeV range gamma rays also require a significant amount of shielding, leading to the irradiation facilities being very large, usually requiring their own building with elaborate shielding and convoluted conveyor systems to safely move the product through the high radiation area. It should be noted also that the gamma rays mostly go through the material without loosing much energy, i.e., without creating much ionization. On the positive side, irradiation sources are inexpensive, stable and require no power to produce the radiation. But while the source itself is inexpensive, the irradiation facility itself is expensive-often costing a million dollars or more. Further, due to the nature of the shielding requirements for radioisotopes, the use of gamma irradiation usually requires a completely separate facility from the manufacturer or distributor and thus results in additional expenses associated with shipping, loading, and packing the materials being irradiated. On top of all this, add the burden of meeting US Nuclear Regulatory Commission and associated state regulatory bodies rules for handling radioactive material.
E-Beam irradiation has several major drawbacks as well. The accelerators are expensive (usually in the million to several million-dollar range) and are fairly big requiring a large room or separate building. Further, unlike gamma irradiation that can penetrate through fairly thick materials (even metals), electrons only travel a short distance in most products. For example, a typical e-beam may only penetrate xc2xc inch (6 mm) in hamburger meat, and is only effective near the surface of materials composed of heavier atoms such as steel. This lack of penetration depth does lead to a benefit in that it may require less shielding if there is not much secondary x-ray production, but the limitations prevent the technology from being useful in many cases. E-beam technology is also usually part of a separate facility as well, creating the same types of transport problems as gamma facilities. Similarly, accelerators must be licensed with the states and are carefully controlled as one of the more dangerous electronic radiation producing products available.
It is also possible to have electrons from an accelerator shine on a heavy metallic target to produce high-energy x-rays or low-energy gamma rays that can in turn be used much in the same way as gamma irradiation from radioisotopes. Unfortunately, the percentage of e-beam energy converted into x-rays energy is only about 1 percent and the overall efficiency is much less than that. Thus, an e-beam x-ray system could be considered the worst of both worlds in that now heavier shielding is required with a much more expensive and inefficient source. A full-scale commercial irradiation facility built on this principle would pretty much require its own separate power plant. With the source being so inefficient that the technique is not economically viable except as an occasionally used add-on feature to an otherwise useful e-beam system.
Therefore, in light of all these problems, a need exists for a device that: (1) is small enough to be integrated into the sites where they are needed; (2) achieves an optimal penetration depth for the product being treated; (3) is safe enough for use by an average person; (4) uses available power efficiently, and (5) is low in cost.
Low energy x-rays appear to meet most of these requirements since they can be tuned so that a maximum amount of x-ray energy is absorbed in a given product. X-ray tubes and power supplies are small and inexpensive and can be made in a wide variety of sizes. Television sets are one example of small economical x-ray producing device since they contain the high voltage supply, vacuum tube and other components that are necessary at very low cost, but use shielding to minimize x-ray emissions.
A traditional x-ray tube is made of a glass or ceramic envelope and is evacuated to a high vacuum. The envelope usually has an x-ray transparent window, typically made of beryllium, aluminum, or glass. The x-ray tube may have x-ray shielding, cooling, and high voltage insulation incorporated into its design as well. The tube has a filament at one end that is intensely heated so that it easily supplies electrons when a high voltage potential is applied between it and the anode. The anode is typically a large block of metal that normally is copper (due to its heat conduction), with a different target material often brazed to the surface that the electrons strike. The vacuum x-ray tube requires two power supplies: a DC power supply for the filament heating which typically operates at low voltage (0-10 volts typical) and a few watts of power; and a second power supply that provides a high voltage (5-200+ kV) DC supply that may range in power from a few watts to 100 kilowatts or more.
Traditional x-ray tubes, however, still suffer from a number of known problems associated with efficiency. When electrons hit the target material of the x-ray tube, they loose the energy they gained from being accelerated by the high voltage electrical potential existing between the filament and the target anode. Through scattering and ionization, the electrons lose energy by transferring some of it to the atoms in the anode target material. For each scattering and ionization event, x-rays and lower energy light are emitted, creating a spectrum of energy that is made up of a continuum of x-rays given up through scattering, and characteristic x-rays of the target material. The efficiency of the conversion of electrical energy to x-ray energy is sometimes expressed by a simple empirically derived formula of the form Ex=E*kZVx where Ex is the x-ray energy, E is the electrical energy, k is a constant, Z is the atomic number of the target, V is the voltage, and x is a power generally accepted to be a little less than 2. By using a higher atomic number target material or higher voltage, it is possible to raise efficiency. Tungsten is a very popular target material for this reason, along with its high melting point and reasonably good thermal conductivity. Other heavy atoms have too low a melting point to be optimal in high-energy x-ray tubes. A tungsten target tube operated at 50 kV potential is approximately 0.7% efficient at converting the energy going into the tube to x-ray energy. When one includes the power supply efficiency, the overall energy efficiency for generating x-rays is less than 0.5%, and then the x-ray beam is further reduced by the window diameter or by collimators that typically allow less than one percent of the x-ray flux to be utilized. This combination of factors results in an effective use of the energy applied to the x-ray tube of less than 50 parts per million (0.005%). The result of these inefficiencies is x-ray tubes and power supplies that are very large and expensive and nearly all of the energy applied becomes waste heat. A small cabinet system that holds less than a cubic foot of material would require a 500-kVA transformer, which is a typical size transformer for an entire small business. Ultimately this wasteful use of energy limits who can practically own and operate x-ray systems for vital uses such as in medical imaging equipment, and makes x-ray tubes unfeasible for certain new applications such as the sterilization of food, medical utensils and products, and countless other beneficial applications of x-rays.
In addition, traditional x-ray tubes, as is also the case with common light bulbs, suffer from frequent filament failure. In both x-ray tubes and light bulbs, the filament is usually tungsten or a tungsten alloy. Over time the tungsten is vaporized, weak spots form, and eventually it breaks. Much of the design improvements over the past 100 years have been directed toward ways of improving filament life through better materials, better cleanliness, and the use of higher vacuum. While filament life has improved, tube life times are typically in the hundreds of hours when operated at anywhere near their peak voltage and current specifications. A side affect of the improvements has been to dramatically increase the manufacturing cost.
The traditional x-ray design has also been driven mostly by the x-ray imaging industry, either medical or industrial, leading designers to develop x-ray tubes with very small focal spots on the anode where the electron beam strikes. While this is a very desirable trait for imaging, it is not desirable when a broad beam source is needed for such applications as sterilization of materials, food irradiation, or x-ray fluorescence. The standard x-ray tube design is inherently a point source design and broader beams are achieved by using larger side windows or end window tube designs that have tighter anode to window geometries allowing for a wider angle exit path. The tube still must be moved farther away from the target being irradiated in order to cover larger areas. The incident dose rates drop with the square of the distance from the source, making the traditional designs even less efficient when a broad beam is required.
It has been known for much of this century that a lamp filled with low-pressure vapors will give off x-rays when a high voltage is applied across it, and during the past few decades there has been a lot of experimental and developmental work on flash x-ray or plasma pinch x-ray devices. They produce x-rays through scattering and electron excitation of the vapor and electrodes as well as the plasma pinch effect that occurs when the magnetic field created by the arc collapses. Flash x-ray devices consist of an x-ray tube filled with a low-pressure vapor and a high voltage capacitive discharge power source. Flash x-ray tubes are generally used for taking high-speed x-ray radiographic images in applications such as ballistics. Their power supply topology limits both their frequency and power, limiting their usefulness as a general source of x-rays. Plasma pinch devices, of which the flash x-ray tube is the simplest version, are being studied intensively as a means of compressing nuclear fuel for fusion. Several very high power devices have been produced but the design of their power supplies have still limited them to pulse operation mostly due to the design goal of igniting a plasma with a single pulse and then maintaining it without additional pulses. To date, the power supplies for these devices consist of a high voltage DC power supply that charges high voltage capacitors, and has a switching mechanism to discharge the capacitors through the tube. The pulse can be as short as tens of nanoseconds to several microseconds in duration. The recharge and cycle rates of the capacitive discharge systems are very slow, typically less than ten per second. Faster types can be made, but are usually lower in power. Both the speed and total power limitations are inherent to the charge-discharge cycle of capacitors. This makes flash x-ray unsuitable for medium and high power continuous operation.
What is important about flash x-ray devices and their cousins, laser ablation x-ray sources, is that both have been shown experimentally to have efficiencies that are, when designed properly, four times higher than a traditional x-ray tube, possibly more. Therefore, a need exists for a new way of driving the flash x-ray device that would allow for high continuous power output at high efficiency to meet the needs of the irradiation application. Much in the same way that the world is converting to fluorescent lighting because it is inherently more efficient than tungsten lighting, a need exists for a fluorescent x-ray system.
Although fluorescent x-ray tubes and power supplies have not been commercially developed for purposes of irradiation, some of the principles underlying the present invention have been used in flash lamps and neon lights. A flash lamp is usually designed to emit a bright flash of light or operated at a higher pulse frequency whereby it can look like it is on constantly to the human eye. A neon light operates at line frequency (60 Hz in the US) or with some newer supplies at 20 kHz or more. Either tube is made of glass or quartz and has two electrodes, which are commonly made of tungsten, predominantly for its high melting point and thermal conductivity. The tube is filled with a vapor that may be at several times atmospheric pressure (1 atm.=760 torr) to 20 torr or less. In order to produce free electrons, a high voltage trigger pulse is usually used to ionize the gas. Then it is operated at lower voltage to produce light. With a large amount of vapor present, the vapor becomes very conductive and effectively shorts out as an arc of electricity passes through it. Traditionally, however, the vapor density is so high that the electrons cannot be accelerated to a high enough potential between scatter events to ionize the inner shell electrons or produce x-rays from the scattering. In fact, the normal operating voltage of flash lamps is only high enough to excite electrons in the outer shells that end up emitting light in the visible, UV, or IR wavelengths. Similarly, neon lights typically have power supplies capable of 9 kV or more, but due to the high fill pressure only a few low energy x-rays are produced. Even the higher voltages are typically so low that the few low energy x-rays that may be produced would be absorbed by the glass envelope. In its simplest form, the flash lamp power supply will consist of a circuit to charge a capacitor that discharges when switched on to both trigger and flash the tube.
In continuous operation, a trigger transformer may be used to produce a high voltage arc to start the tube, then a lower voltage supply, which may be DC, or pulsed DC or AC at a variety of frequencies, will be used to drive the tube. A neon light will have a ballast and step-up transformer typically with two secondary windings to generate positive and negative high voltage. The newer high frequency resonant supplies for neon lights convert the line voltage to DC, then produce high frequency ( greater than 20 kHz) AC with a resonant inverter and then use a step-up transformer. The front end of these power supplies up to the transformer is also very similar to the electronic ballasts used in fluorescent lighting. These tubes are available in many sizes and shapes, which are conceivably adaptable to fluorescent x-ray tube applications.
Some of the above-mentioned systems use pulsed DC supplies that rely on capacitive discharge. These supplies are frequency limited by the charge and discharge cycles of the capacitors that also limit the life of the supply. Many capacitors also discharge slowly compared to potential speed of an arc, and so are relatively inefficient at producing x-rays. Resonant supplies are commonly used in fluorescent lighting and resonant supplies with a high voltage transformer are available for neon lighting. Even the first stages of many high voltage power supplies have incorporated resonant inverter technology. These high frequency devices can have smaller and more efficient transformers since they move less power per half sine wave, so the overall supply is smaller and more efficient.
In light of all this, a need exists for a new type of x-ray tube that is lower in cost, more efficient, and illuminates a broader area than current technology, while eliminating the troublesome filament. To achieve these goals it is necessary to integrate new and novel approaches for increasing the efficiency of x-ray production, and design a new power supply accordingly to create a design for a new class of x-ray tube and power supply.
Accordingly, the present invention provides a fluorescent x-ray tube and power supply system that overcomes the problem associated with known sources of x-rays.
The device in accordance with an embodiment of the present invention consists of a fluorescent x-ray tube powered by a resonant high voltage power supply that is suitable for use in an x-ray irradiation device or other device requiring an x-ray source. The fluorescent x-ray tube consists of an envelope made of quartz or other suitable non-conductive material, with electrodes mounted on opposing sides, and filled with a low-pressure vapor. The high voltage resonant power supply generates high frequency alternating current (AC) or direct current (DC) pulses. Arcs are formed between the electrodes when the potential reaches a high enough voltage, usually at or near the power supplies peak voltage. As the electrons move through the tube they periodically scatter off vapor atoms or molecules in their paths, ionizing the vapor, and losing some or all of their energy in the process. Scattering and ionization result in continuum and characteristic x-ray production. Free electrons and ions will be accelerated by the potential between the electrodes and periodically scatter off vapor atoms until they strike an electrode and produce additional x-rays. The arc in the tube also creates a magnetic field. This field collapses when the arc stops, creating a plasma pinch that also leads to x-ray production.
Another aspect of the present invention involves the improvement of the efficiency gain in the fluorescent x-ray tube. The efficiency gain in the fluorescent x-ray tube is a direct result of the excitation of the atoms that comprise the vapor. Once the pressure in the tube is low enough to sustain a high voltage arc, the mean free path for the electrons is long enough for the free electrons to gain enough energy between collisions to produce x-rays when they are scattered. This also leads to multiple acceleration zones and therefore multiple x-ray producing interactions along the length of the arc path. The plasma pinch phenomenon at the end of an arc is also responsible for a great deal of the radiation output. In one embodiment of the invention, the efficiency gain is at least five times that of a standard vacuum x-ray tube. The fluorescent x-ray tube also operates as a cold cathode device using free electrons from the excited vapor or electrodes thus eliminating the need for the fragile filament.
The operation of the fluorescent x-ray tube is similar in many ways to the most modem designs for fluorescent lamps, neon lights, or flash lamps, except that the vapor pressure is much lower and the voltage much higher. In the operation of these normal everyday lamps, only the outermost electrons from the vapor atoms are excited, so that s they produce light mostly in the UV, visible, and infrared regions. Flash x-ray systems are also fundamentally similar since they use vapor arc discharges to produce x-rays. Flash systems typically have a capacitive discharge-type supply that is generally suitable to pulsed or low frequency, (typically less than 1000 Hz), operation only. To improve the efficiency while reducing size and cost of the power supply, the present invention incorporates high frequency resonant inverter technology into the supply with the addition of a high frequency high voltage transformer. The inherent difficulty to adapting this technology directly to the fluorescent x-ray tubes is designing transformers that are small enough to operate at high frequency, but big enough to incorporate the insulation needed for the high voltage. In addition, making supplies that can deliver more power is a challenge. In order to make this x-ray source useful for an irradiation application, it is necessary to make a supply that is capable of delivering kilowatts of power instead of a few hundred watts, and generating voltages of 50 kV or more. To meet these requirements and overcome the problems with known power supplies the present invention provides in a new class of x-ray source that can produce x-rays with high efficiency and can be operated in a continuous fashion.
In an embodiment of the present invention, it is envisioned that many different vapors will be desirable and could be used within the x-ray tube to satisfy the need for x-rays of different energies under different circumstances. Each element of the periodic table, when ionized, is capable of giving off different characteristic wavelengths or energies of photons, including x-rays. In addition, it must be kept in mind that higher energy x-rays have greater penetration power, which is beneficial for penetrating thicker, higher density, or higher atomic number materials. It is known that the output efficiency of a vacuum x-ray tube is proportional to the atomic (Z) number of the anode material. Likewise, the efficiency of the fluorescent x-ray tube will also have a proportional relationship to the atomic number(s) of the vapor constituents in addition to the electrode element(s).
In addition, it will be appreciated by those familiar with x-ray devices that the fluorescent x-ray tube may be constructed of various materials or have various windows installed that are relatively transparent to the radiation energy required by a specific process or application. The electrodes may also be composed of materials that are common to the art, and may be selected for their characteristic x-rays, atomic number, melting point, thermal conductivity, electrical conductivity, ionization potential, coefficient of expansion, and various other relevant properties.
The fluorescent x-ray tube of the present invention offers vastly improved x-ray production efficiency, a lower production cost for both the x-ray tube and power supply, less heat generation, and it is designed to eliminate the troublesome filament common to existing designs. In addition, the present invention easily configured as a broad beam source, since x-rays are emitted along the entire arc path length. This allows large materials to be placed nearer to the x-ray source, thus minimizing spatial transmission losses in comparison with traditional point source x-ray tubes. In an alternate embodiment, the fluorescent x-ray tube of the present invention can be collimated and/or designed with a short arc gap similar to typical commercially available arc lamps for imaging applications. The expensive DC power supply used in traditional x-ray tubes is replaced with a much lower cost resonant supply, and the x-ray tube itself may be constructed in a much less expensive manner than traditional vacuum x-ray tubes, thus making the invention useful for otherwise cost prohibitive uses.
The fluorescent x-ray tube of an embodiment of the present invention is well suited to the product irradiation application due to its high efficiency and broad beam capabilities. Material that is to be irradiated can be positioned in close proximity to receive x-rays from the tube, in a variety of possible configurations.
In alternate embodiments, the packaging of the irradiation device can also have several embodiments. In one embodiment, the packaging of the device may be a cabinet-type device similar to a microwave oven where product is place inside in order to be treated. In another embodiment the device is built over or around a material conveyance apparatus for continuous or batch treatment much like an airport x-ray scanning system. In a third embodiment, it is a flow through device where the product, such as liquids or air conveyed materials, flow through an area being irradiated. Shielding and safety interlocks are added as needed to protect operators of the equipment and bystanders.
A fluorescent x-ray tube is beneficial for other typical x-ray applications as well, including but not limited to x-ray fluorescence, medical and industrial imaging, medical treatment, and x-ray lithography.